(1) Field of the Invention
This invention relates to a flash fixing apparatus for fixing an unfixed toner image, which has been formed on a base material by an electrophotographic printer, for example, by light radiated from a flash lamp.
(2) Description of the Prior Art
For example, an electrophotographic printer is usually equipped with an original document exposing unit, a developing unit, a transfer unit and a fixing unit. A toner image formed by development is usually transferred onto a base material, followed by its fixing by the fixing unit so as to form copied marks.
As fixing systems for such electrophotographic images, there have heretofore been known the gas-dependent fixing method, pressure-dependent fixing method, thermal fixing method, etc.
However, the gas-dependent fixing method is accompanied by problems that it requires a complex apparatus and is difficult to practice. In the pressure-dependent fixing method, a toner image is fixed by pressing the toner of the image directly while holding the toner image in contact, for example, with a roller. This method is therefore accompanied by a problem that certain distortion tends to occur in copied marks. On the other hand, the thermal fixing method is most widely employed where base materials are made of paper. This thermal fixing method is also accompanied by a drawback that some distortion tend to occur in copied marks, because heat is usually transmitted to a toner image so as to fix it while holding a fusing roller in contact with the toner image. The base material may be deformed, that is, may be caused to undergo so-called waving upon fixing, especially, where the base material is a plastic film having low heat resistance.
For the reasons described above, a fixing method making use of light from a flash lamp, namely, the flash fixing method has been finding more and more utility recently. According to this flash fixing method, it is possible to achieve prompt fixing without need for any warm-up time which is indispensable in the fixing method making use of fusing rollers. Further, a toner image on a base material can be fixed without bringing anything into contact with the toner image and the toner image can hence be fixed without lowering its resolution obtained by its development. As a further merit of the flash fixing method, it permits selective use of base materials having various different thicknesses. In this flash fixing method, light radiated from a flash lamp is absorbed by toner and is then converted to heat, thereby achieving the fixing of the toner. Since the radiation time of the light is extremely short, namely, 0.1--several milliseconds, it is possible to effect good fixing without causing thermal deformation (waving) even when the fixing is effected on a base material having relatively low heat resistance such as a plastic film.
When photocopies of maps, drawings which require long-term storage, or the like are made with toner, it is not recommendable to use paper sheets as base materials because such base materials are susceptible to expansion and shrinkage by moisture and the dimensional accuracy of the resultant fixed marks varies in accordance with the conditions of storage environment of the base materials and can hardly be maintained stably. For these reasons, special paper sheets with plastic or metallic core materials, each of which is resistant to expansion and shrinkage by moisture and is incorporated in the form of a sandwich, have been finding more and more utility.
When a photocopy is formed by toner, it may be desired in some instances to once discharge, in an unfixed state, a base material with a toner image transferred thereon from an electrophotographic printer without immediate fixing of the toner image. The unfixed toner image is then corrected as needed, and the thus-corrected toner image is fixed to obtain a final visible image. In order to meet such a demand, extensive research and development work is now under way with respect to flash fixing apparatus which are separate from electrophotographic printers and are employed exclusively for fixing purposes.
In such a flash fixing apparatus, conveyor means is required to convey each base material from its inlet part to its outlet part. Since the base material to be conveyed carries an unfixed toner image thereon, it is necessary to convey the base material smoothly without damaging the toner image. As conveyor means for such a base material, means which makes use of belts are simple and preferable. This belt-type conveyor means is however accompanied by a drawback that a base material is susceptible of curling along its edge areas and is hence bent or wrinkled especially when a base material in the form of a thin film is conveyed.
Since heating by light from a flash lamp is effected instantaneously, the base material may not be heated to any high temperature enough to cause the toner to adhere the base material with sufficient strength. This trouble occurs more frequently as the environmental temperature becomes lower, for example, as in winter.
If the energy of light from a flash lamp is increased, a contradictory phenomenon occurs that the toner is caused to scatter around at the moment of flashing and is not fixed sufficiently in contrast to the intention.